A polymer film of typically cellulose ester, polyester, polycarbonate, cyclo-olefin polymer, vinyl polymer or polyimide is used in silver halide photographic materials, retardation films, polarizers and image display devices. Films that are more excellent in point of the surface smoothness and the uniformity can be produced from these polymers, and the polymers are therefore widely employed for optical films. Of those, for example, cellulose ester films can be directly stuck to most popular polarizing films formed of polyvinyl alcohol (PVA)/iodine in on-line operation, because they have suitable moisture permeability. Accordingly, cellulose acylate, especially cellulose acetate is widely employed as a protective film for polarizers.
On the other hand, when a transparent polymer film is used in optical applications for retardation films, supports of retardation films, protective films of polarizers, and liquid crystal display devices, then it is an extremely important element to control the optical anisotropy of the film for determination of the performance (e.g., visibility) of display devices. With the recent requirement for broadening the viewing angle of liquid crystal display devices, it has become desired to improve retardation compensatory capability, and it is therefore desired to suitably control the in-plane retardation (Re, this may be hereinafter simply referred to as “Re”) and the thickness-direction retardation (Rth, this may be hereinafter simply referred to as “Rth”) of the retardation film to be disposed between a polarizing film and a liquid-crystal cell. In particular, since a transparent polymer satisfying |Rth|/Re<0.5 is not easy to produce, it is desired to develop a simple method for producing it.
On the other hand, as a production method for a polymer film, a continuous production method is disclosed, which comprises adhering a thermoshrinkable film to a polymer film, then thermally stretching it, and thereafter peeling off the thermoshrinkable film (for example, see Patent Reference 1 and Patent Reference 2). Examples in these references clarify that the polycarbonate films produced according to the method satisfy the condition of |Rth|/Re<0.5. However, the method is problematic that it consumes a large quantity of thermoshrinkable films and that the quality of the obtained films is not uniform. The problems are especially remarkable with polymers of high elasticity, such as those of cellulose ester.
Patent Reference 1: JP-A-5-157911
Patent Reference 2: JP-A-2000-231016